This invention generally relates to wireless networks and specifically to wireless local area network (“WLAN”) devices.
Wireless local area networks (“WLAN”) allow network devices, such as computers, to have network connectivity without the use of wires. Network connections may be established via, for example, radio signals. A wireless access point (“AP”) may comprise a wired Internet or Ethernet connection and a radio communication circuitry capable of transmitting data to and receiving data from another compatible wireless device. The AP may provide Internet and/or network connectivity to such other network devices (e.g., computer) called stations (“STA”) by transmitting and receiving data via radio signals.
A variety of industry standards exist that govern the implementation of WLANs. Examples of such industry standards comprise the IEEE 802.11a, 802.11b and 802.11g protocols. The 802.11a and 802.11g protocols utilize the Orthogonal Frequency-Division Multiplexing (OFDM) communication method where the data to be transmitted is split into multiple parallel data streams and each parallel data stream transmitted simultaneously over narrow sub-channels that togther form the full channel bandwidth of 20 MHz. The 802.11b protocol utilizes the direct sequence spread spectrum (“DSSS”) communication method. DSSS enables communication between two devices by splitting into several parts each byte of data to be transmitted and sending each part concurrently on different frequencies across a 24 MHz-wide spectrum.
In many WLANs, each wireless device (AP or STA) has a single signal path for transmission, a single signal path for reception, and a single antenna. Some WLAN devices may have multiple antennas, but only a single signal path for reception and a single signal path for transmission; the most favorable antenna may be connected to transmit and receive signal chains through a switch (a technique known as antenna switched diversity). In the following, a single-antenna transceiver is defined as a transceiver that has a single signal path for reception and a single signal path for transmission. A single-antenna transceiver also may be defined as a transceiver with multiple antenna structures that may be selectively connected to the signal paths with a control switch. A multi-antenna transceiver is defined as a device having multiple transmission signal paths and multiple receive signal paths in addition to a plurality of radio structures that may be connected to the signal paths. The performance of such WLAN systems is determined by data rates achieved between an AP WLAN transceiver and any STA WLAN transceivers communicating with the AP. There exist a wide range of possible operating conditions between an AP and each STA associated with the AP. Typically, the maximum achievable data rates between the AP and a given STA decrease as the distance between the AP and the STA increases.
The rate at which data is transferred (“data rate”) between an AP and each STA associated with the AP may be raised by increasing the number of antennas associated with each wireless device in the system. For instance, a system comprising an AP with multiple antennas and an STA with multiple antennas may have a higher data rate than a system comprising an AP with a single antenna and an STA with a single antenna. The multiple-input antennas and multiple-output antennas (“MIMO”) are part of a design that attempts to achieve a linear increase in data rate as the number of transmitting and receiving antennas linearly increase.
WLAN systems employ MIMO “space-time” coding, wherein a transmission signal is split in the time domain and the signal is distributed across the multiple transmitting antennas in space. When combined with a multi-carrier modulating scheme, such as Orthogonal Frequency Data Modulation (“OFDM”), the encoding technique is referred to as “space-time-frequency” coding. A multiple-antenna receiver may receive and process a space-time encoded signal or a space-time-frequency encoded signal to determine the data transmitted from each transmitting antenna.
A multiple-antenna WLAN transmitter (e.g., an AP) produces a set of signals that each pass through separate signal paths for digital modulation, analog and radio frequency processing and wireless transmission over the antennas. These paths are determined largely by the various positions of the transmitting and receiving antennas. Some paths may be at a disadvantage to other paths due to undesirable factors, such as signal noise. Paths that have less interference and greater signal clarity are said to have a relatively high signal-to-noise ratio (“SNR”). Thus, paths with greater SNRs are preferred over paths with lesser SNRs.
Since single-antenna WLAN devices (e.g. STAs) are not compatible with multiple-antenna WLAN devices, systems solely comprising single-antenna WLAN devices cannot take advantage of the vast improvement in performance realized by systems solely comprising multiple-antenna WLAN devices. A performance-improving technique for a multiple-antenna WLAN device communicating with a single-antenna WLAN device is desirable.